1. Field of the Invention
The present invention relates to a free space optics communication apparatus which uses a light beam emerging into the space to perform wireless transmission of information.
2. Detailed Description of the Related Art
In a conventional free space optics communication system, a free space optics communication apparatus on the transmission side modulates a transmission signal into a light signal which then emerges as a light beam toward a free space optics communication apparatus on the reception side provided opposite thereto. The emerging light beam travels through the atmospheric space and is received at the free space optics communication apparatus on the reception side (another apparatus). The other apparatus demodulates the light signal transmitted through the light beam from the transmission side. In this manner, information signals are transmitted and received.
In the free space optics communication system as described above, the path of the light beam may vary due to fluctuations of the atmosphere or the like. In addition, the emerging direction of the light beam may vary since slight deformation readily occurs in a facility where the free space optics communication apparatus is installed, for example the rooftop of a building, due to a change in temperature.
When these external factors cause the light beam to arrive at varying positions on the reception side, the level of reception is reduced at the free space communication apparatus on the reception side, and at worst, communication is interrupted.
In addition, in a case that deformation occurs in a facility where the other apparatus on the reception side is installed, and the apparatus on the reception side is not disposed in a direction appropriate for the incident light beam, the level of reception is reduced and the reception may be impossible.
To solve the problems, a proposed free space optics communication apparatus is configured to emerge a light beam such that the reception side receives the light beam of large diameter to ensure a stable level of reception on the reception side even when the beam path or the emerging direction of the beam varies.
Also, the apparatus on the reception side is designed to have a certain size of light-receiving angle (an angular range in which light can be received).
FIG. 12 shows an exemplary conventional free space optics communication system. In FIG. 12, a free space optics communication apparatus 50 on the transmission side is shown on the left, while a free space optics communication apparatus 71 on the reception side (another apparatus) is shown on the right. On the transmission side, a light beam emitted from a light source 72 is converted by an optical system 73 into a light beam 76 which consists of substantially parallel rays with slight divergence. On the other hand, on the reception side, an optical system 75 is designed to have a light-receiving angle slightly divergent as compared with parallel rays. The optical system 75 converges the light beam which is received by a light-receiving element 74.
In general, to change the emerging angle or the receiving angle of a light beam in such a free space optics communication system, the optical system, the light source, or the light-receiving element is mechanically moved to change their relative positional relationship. In this case, the relative positional relationship between the light-emitting element 72 and the lens 73 is changed to vary the divergent angle. Similarly, on the reception side, the relative positional relationship between the light-receiving element 74 and the lens 75 is changed to vary the light-receiving angle.
In free space optics communication, bi-directional simultaneous communication is typically performed. A free space optics communication apparatus has a transmitting unit and a receiving unit formed therein. When the apparatus has a transmitting optical axis and a receiving optical axis coincident with each other, the apparatus can be provided with an automatic tracking function of detecting the incident direction of light transmitted from another apparatus and matching the optical axis direction of the light beam from the other apparatus with the transmitting and receiving optical axes of the apparatus itself. In this case, it is necessary to provide a means for changing the direction of a transmitting or receiving beam.
Possible methods of changing the direction of a transmitting or receiving beam include changing the angle of the entire apparatus or the optical system, providing an angular change for the beam by using a mirror in the optical system, providing a positional change between the lens and the light-emitting element or the light-receiving element, or the like.
FIG. 13 shows an example of the methods. A light beam emerging from a light-emitting element 81a at a divergent angle 83 passes through a lens 82 and then emerges therefrom as a light beam 85 consisting of substantially parallel rays.
A change in the beam emerging direction can be realized by changing the relative position between the light-emitting element (light source) 81a and the lens 82. When the light-emitting element 81a is shifted to a position 81b in FIG. 13, a light beam 84 incident on the lens 82 emerges into the space from the lens 82 as an emerging beam 86. The direction of the beam 86 is different from the direction of the aforementioned beam 85.
These methods are used not only to realize the automatic tracking function but also to align beam directions when the apparatus is installed.
Description is made for another structure which changes the relative positional relationship between the light-emitting element or the light-receiving element and the optical system with reference to FIG. 14.
At the top in FIG. 14, a light beam is emitted from the end surface of a light-emitting element 61 at a divergent angle 65. The divergent angle of the light beam is reduced by a lens 62 and then the light beam is sent into the space at a divergent angle 68 which is substantially collimated. For reducing the beam divergent angle to increase the power density of light received at a receiving unit, the light-emitting element 61 is moved on an optical axis in a direction away from the lens 62 as shown at the bottom in FIG. 14. Of output light from the light-emitting element 61, a light signal component incident effectively on the lens 62 in an angular range 66 results in a beam emerging angle 69 smaller than the angle 68. The light beam emerging from the lens 62 can become more collimated luminous flux.
However, since the divergent angle 65 of the light output from the light-emitting element 61 is constant, rays 67 which are not effectively incident on the lens 62 are ineffective to cause a reduction in light power transmitted into the space through the lens 62.
If the relative positional relationship between the light-emitting element 61 and the lens 62 is not changed and the curvature of the lens 62 can be changed, the divergent angle can be controlled effectively. However, it is difficult to form a lens or a lens unit of which the curvature can be changed, and the structure thereof is complicated. In addition, a precise mechanism and accurate control are required to avoid variations in the emerging direction of light.
Typically, in the free space optics communication, the diameter of a received beam needs to be minimized at a reception point in order to obtain sufficient light-receiving power. However, it is difficult to form a free space optics communication apparatus which can freely change the beam emerging angle or the light-receiving angle for the reasons as described above.
The shape of a beam may need to be controlled as well as the beam divergent angle. In the free space optics communication apparatus, typically, the light-receiving angle is desirably set to be large on the reception side as described earlier. If a strong light source such as the sun, a search light or the like is present behind the apparatus on the transmission side, the strong light is incident on the light-receiving element to prevent the intended light beam from being received clearly.
FIG. 15 shows how the apparatus on the transmission side is viewed from the apparatus on the reception side. A strong light source 43 such as the sun, a search light or the like, different from an inherent light source, partially enters a reception range 42 including an apparatus 41 on the transmission side viewed from the apparatus on the reception side.
Especially, the sunlight has much larger light power (energy) than the light power of a light beam from the apparatus 41 on the transmission side. Thus, a communication failure may occur if even a small amount of the sunlight enters the light-receiving range.
The conventional free space optics communication apparatus using a lens takes measures against the incidence of the strong background light as mentioned above, for example by setting a smaller light-receiving angle or changing the relative positional relationship between the light-receiving element and the lens 62 according to the circumstances as shown in FIG. 14. Such measures, however, cause a problem that stable communication is unlikely to be ensured in the presence of the fluctuations of the installation environment or the like.
On the other hand, for controlling the shape of a beam pattern or the like, it is necessary to provide an optical system responsible for that control in addition to the optical system which controls the beam emerging angle or the light-receiving angle.
In addition, the shape (and the power density distribution) of a beam pattern subjected to control is uniquely determined by the provided optical system and thus cannot be arbitrarily changed.
Furthermore, when a change in the beam emerging direction or the light-receiving direction is intended, the structure shown in FIG. 13 presents a problem that the light power output from the light-emitting element 81a is not effectively used, similarly to the method of controlling the beam emerging angle or the light-receiving angle.
Specifically, a component 87 shown in FIG. 13 is not effectively incident on the lens 82 and is unnecessary since it is not transmitted into the space. Also, if the component 81 is used as a light-receiving element, the effective light-receiving angle 86 is smaller than the angle 85 which corresponds to the original light-receiving angle, thereby reducing the efficiency of reception.
Each of the optical systems formed for the respective purposes is a solid-state lens, a prism or the like. It is difficult to change the optical systems separately or simultaneously at a high speed.
A conventional free space optics communication apparatus which uses a light beam to perform point-to-multipoint information transmission radiates light toward a plurality of other apparatuses at remote locations to cover the entire range in which the other apparatuses are present as in radio communication. In this case, the transmission side needs to output light at a high level to allow the individual other apparatuses to receive light with sufficient levels.
In the free space optics communication apparatus, however, the transmission side has a limited output level of light from the viewpoint of the lifetime of a light-emitting element or the like. It is thus impossible to radiate light at a sufficient level in the entire range in which the other apparatuses are present.
In general, the free space optics communication apparatus which uses a light beam to transmit information to a plurality of other apparatuses at remote locations has a plurality of mirrors which reflect and send emerging light from the apparatus itself toward the plurality of other apparatuses or reflect and take light sent from the plurality of other apparatuses into a light-receiving section thereof. Since each of the plurality of mirrors is set at an angle appropriate for transmission and reception of light between the apparatus and each of the other apparatuses, the use of the mirrors enables efficient transmission and reception of light in the free space optics communication apparatus for point-to-multipoint communication.
As described above, the free space optics communication apparatus needs to have a margin for a certain deviation of the optical axis of received light transmitted by any of the other apparatuses from the optical axis of the light-receiving section thereof to avoid a deteriorated S/N ratio due to such a deviation caused by fluctuations of the air or the like.
The margin for a deviation is increased by setting a larger divergent angle of emerging light on the transmission side. Typically, such a large divergent angle of an output light beam is achieved by providing a concave surface for the plurality of mirrors which reflect and send emerging light to the plurality of other apparatuses or reflect and take light transmitted from the plurality of other apparatuses into the light-receiving section.
FIG. 16 shows a conventional free space optics communication apparatus. Reference numeral 901 shows a transmitting circuit, 902 a light-emitting element, 903 a polarization beam splitter which separates transmission light and received light, and 904 a concave mirror unit which allocates light to other apparatuses. Reference numeral 905 shows a light-receiving element, and reference numeral 906 shows a receiving circuit.
The transmitting circuit 901 converts a signal to be transmitted into a signal which can be electro-optically converted. The light-emitting element 902 converts the signal into light which then emerges as a light beam. The transmission light emerging from the light-emitting element 902 passes through the polarization beam splitter 903, is reflected by the concave mirror unit 904, and is directed toward each of the plurality of other apparatuses.
On the other hand, a light beam transmitted from any of the other apparatuses is reflected by the concave mirror unit 904, and sent to the polarization beam splitter 903. The light beam reflected by the polarization beam splitter 903 is converted into an electric signal by the light-receiving element 905, and information included in the signal is received by the receiving circuit 906.
The concave mirror unit 904 has the structure as shown in FIG. 17. Specifically, four concave mirrors 910 are arranged as in FIG. 17, in which each of the concave mirrors 910 is supported to be movable at any angle.
In the prior art, however, the number of the mirrors in the free space optics communication apparatus is determined in the manufacturing stage of the apparatus. Thus, when the apparatus is actually installed, the number of mirrors is not always consistent with the number of other apparatuses at that point.
If the number of the other apparatuses is smaller than the number of the mirrors, a, certain number of mirrors are not used corresponding to the difference between them. In this case, since light from the light source is always made incident on the mirrors, light sent from the unused mirrors is unnecessary.
In addition, since the mirrors have the same curvatures, problems arise both when the other apparatus is located at a shorter distance and when at a longer distance. For the other apparatus at a shorter distance, received light has a small beam diameter and is incident on the light-receiving element at too high a level, which may cause a failure of the light-receiving element. For the other apparatus at a longer distance, received light has an extremely large beam diameter and a smaller amount of light is received to reduce the margin for rain or the like, in which case communication may be interrupted by a little rain or fog. Thus, the communication range is limited in the prior art.
When the number of other apparatuses is larger than the number of the mirrors, an additional apparatus (apparatus on the transmission side) is installed, or when the number of other apparatuses is smaller than the number of the mirrors, light sent from extra mirrors is unnecessary. In addition, an available communication range is limited.